<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container{
           display: flex;
           height: 500px;
        }
        .sub{
            width: 200px;
            /* height: 500px; */
            background-color: blue;
        }
        .main{
            flex: 1;
            /* height: 500px; */
            background-color: yellowgreen;
        }
        .extra{
            width: 200px;
            /* height: 500px; */
            background-color: steelblue;
        }
    </style>
</head>
<body>

    <div class="container">
        <div class="sub"></div>
        <div class="main"></div>
        <div class="extra"></div>
    </div>
    
</body>
</html>